The invention relates to a hydraulic unit for a cylinder head of an internal combustion engine with a hydraulic, variable valve train that comprises                at least one drive-side master unit,        at least one driven-side slave unit activating a gas-exchange valve,        at least one controllable hydraulic valve,        at least one medium-pressure chamber,        at least one high-pressure chamber that is arranged in the sense of transmission between the associated master unit and the associated slave unit and can be connected to the associated medium-pressure chamber via the associated hydraulic valve, and        a hydraulic housing with a housing bottom part and a housing top part,wherein at least the master unit, the slave unit, the high-pressure chamber, the hydraulic valve, and the medium-pressure chamber in connection with the hydraulic housing belong to the hydraulic unit that can be mounted on the cylinder head.        
Such a hydraulic unit is known from DE 10 2006 008 676 A1 that is considered a class-forming patent. In the hydraulic unit provided there, all essential components required for the hydraulic, variable transmission of raised sections of a cam to the gas-exchange valves are combined into a common hydraulic housing. This is assembled from a housing bottom part in which the components named above are housed and in which the compression chambers extend and a housing top part closing the housing bottom part. The housing bottom part has a very compact construction and the housing top part also involves an essentially flat plate, so that, overall, each of the medium-pressure chambers is limited to a correspondingly small volume.
However, a small-volume medium-pressure chamber can be problematic during the startup process of the internal combustion engine, in particular, for a startup process at low outside temperatures and after a long standstill of the internal combustion engine. This is based on the fact that the hydraulic medium supply of the internal combustion engine still does not deliver sufficient hydraulic medium flow into the medium-pressure chamber during the startup process and only the hydraulic medium volume remaining in the medium-pressure chamber and also contracted at low temperatures is insufficiently large for a complete refilling of a then expanding high-pressure chamber. This problem applies to an increased extent for startup processes that are repeated in a short time sequence, because, in this case, the hydraulic medium consumption from the medium-pressure chamber can be greater than the volume fed back from the hydraulic medium supply of the internal combustion engine. Such multiple startup processes are typical, for example, for taxi vehicles at taxi stands.
Starting from the publication cited above, this problem can also be eased to only a limited extent by expanding the medium-pressure chamber in the direction of the housing top part then equipped with corresponding cavities, because in this case the formation of gas bubbles in the medium-pressure chamber and their intake into the expanding high-pressure chamber was not reliably ruled out.